1. Field of the Invention
The present invention relates to a molecular beam epitaxy (MBE) system capable of forming II-VI column compound semiconductor layers and also to a method of fabricating an optical semiconductor device, using this MBE system.
2. Description of the Related Art
II-VI column compound semiconductors consisting of zinc selenide (ZnSe) or the like are applied to optical semiconductor devices such as laser diodes emitting blue-green light and light-emitting diodes.
Where a semiconductor device is fabricated, using these II-VI column compound semiconductors, the issue of ohmic contacts to p-type layer is still a subject, because it is difficult to obtain a high acceptor concentration. On the other hand, high p-type conductivity is obtained from zinc tellurium (ZnTe) more easily than from various other II-VI column compound semiconductors. Consequently, the use of ZnTe is essential for formation of the above-described ohmic contact.
An example of the optical semiconductor device which uses an ohmic contact made of ZnTe layer and emits blue-green light is shown in FIG. 3. This optical semiconductor device, 100, is a semiconductor laser. For example, this device uses a substrate 110 made from gallium arsenide (GaAs). A buffer layer of GaAs 121 is formed on the substrate 110. II-VI column compound semiconductor layers 120 are formed on the buffer layer 121. One example of this is described below.
Specifically, a buffer layer 122 of ZnSe and an n-type cladding layer 123 consisting, for example, of zinc magnesium sulfur selenide (znMgSSe) are successively formed over the GaAs buffer layer 121. Furthermore, an n-type guiding layer 124 made from zinc sulfur selenide (ZnSSe), an active layer 125 consisting, for example, of zinc cadmium selenide (ZnCdSe), and a p-type guiding layer 126 consisting of ZnSSe are stacked.
A p-type cladding layer 127 made from zinc magnesium sulfur selenide (ZnMgSSe:N) and a p-type layer 128 made from ZnSSe are successively formed over the p-type guiding layer 126.
A contact layer 129 consisting, for example, of a multilayer film of ZnSe/ZnTe and having a superlattice structure and a cap layer 130 made from ZnTe are successively formed over the p-type layer 128. That is, the contact layer 129 and the cap layer 130 cooperate to form a p-type ohmic layer. Thus, the II-VI column compound semiconductor layers 120 are constituted.
Heretofore, the above-described II-VI column compound semiconductor layers 120 have been formed by MBE (molecular beam epitaxy) using an MBE system, for example. In the MBE process, the constituent elements of the II-VI column compound semiconductor layers 120 are evaporated in an ultra-high vacuum and epitaxially grown on the substrate 110, thus forming the layers 121-130.
However, tellurium (Te) is an element having a high vapor pressure and thus stagnant in nature. Therefore, where the substrate placed inside the chamber of the MBE system is isolated by a shutter from a Te cell connected with the chamber, if the temperature of the Te cell is elevated to a level at which growth is possible, then Te goes to the substrate through an unstraight route. For this reason, when the p-type cladding layer under the contact layer containing ZnTe is grown, Te is mixed into the p-type cladding layer. As a result, the hole concentration in the p-type cladding layer decreases. In consequence, the crystallinity deteriorates.
For example, when the hole concentration in the p-type cladding layer consisting of ZnMgSSe is about 3.times.10.sup.17 cm.sup.-3, if about 2% Te is mixed into the selenium (Se) in the p-type cladding layer, then the hole concentration drops to about 1.times.10.sup.17 cm.sup.-3. Also, recombination center due to Te in the n-type cladding layer is observed with photoluminescence (PL) at 77 K.
As a result, the fundamental emission characteristics are affected severely. For instance, the operation current or the threshold current of the formed optical semiconductor device increases, and the intensity of the emitted light decreases.